Streaming volumetric and non-volumetric video

ABSTRACT

A processor system and computer-implemented method are provided for generating a composite video stream which may include a non-volumetric video and a volumetric video, at least part of which is to be displayed in front of the non-volumetric video. The volumetric video may be included in the composite video stream in the form of a non-volumetric representation of the volumetric video, for example inserted into a spatial subregion of the non-volumetric video which may be occluded by the volumetric video during display. The encoding, transmission and decoding may thus not have to be modified to support volumetric video. Signaling data may be provided which may be indicative of the composite video stream containing the non-volumetric representation of the volumetric video. A processor system and computer-implemented method may be provided for rendering the composite video stream using the signaling data.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a national stage entry of, and claimspriority to, PCT/EP2019/085599, filed on Dec. 17, 2019, which claimspriority to European Patent Application EP 18215425.2, filed in theEuropean Patent Office on Dec. 21, 2018, both of which are herebyincorporated in their entirety herein by reference.

FIELD OF THE INVENTION

The invention relates to a method and processor system for generating acomposite video stream for being displayed by a client device, forexample in an Augmented Reality or Virtual Reality environment. Theinvention further relates to a computer readable medium comprisingsignaling data for use in generating the composite video stream. Theinvention further relates to a method and a processor systemrepresenting a client device for rendering the composite video stream.The invention further relates to a computer program for carrying outeither method.

BACKGROUND ART

It may be desirable to transmit a volumetric video and a non-volumetricvideo to a client device for simultaneous display by the client device.

Such a scenario may, for example, occur in the fields of Virtual Reality(VR) and Augmented Reality (AR). Here, VR involves the use of computertechnology to simulate a user's physical presence in a virtualenvironment, while AR refers to the use of computer technology toaugment a user's view of the physical real-world environment byoverlaying a virtual environment over, or in another manner combiningthe virtual environment with the user's view of the physical real-worldenvironment. Typically, VR and/or AR rendering devices make use of HeadMounted Displays (HMD) to render the virtual environment to the user,although particularly in VR, other types of displays and renderingtechniques may be used as well, including but not limited to holographyand Cave automatic virtual environments (recursive acronym CAVE).

VR/AR may be used to render scenes which are represented bythree-dimensional (3D) graphics, e.g., defined as a set of vertices,edges, faces, etc.

However, in many cases, it may be desirable to establish a video-basedrepresentation of a scene in VR. For example, the video may be a videorecording of a real-life scene, or a video obtained byComputer-Generated Imagery (CGI) of a virtual scene, etc. In some cases,the video may represent a panoramic or omnidirectional video which mayprovide a large field of view of the scene, e.g., allowing a user to‘look around’ within the scene. The rendering of such a video-basedrepresentation of a scene may involve displaying the image data of thevideo on the inside of a virtual body, such as a sphere, and thenrendering from a viewpoint within or facing the virtual body. Forexample, in a multiuser communication session in VR, which is alsoreferred to as ‘Social VR’, an omnidirectional video of a tropicalisland may be used as a ‘virtual backdrop’ for the multiusercommunication session by projecting the video onto the inside of asphere and placing avatars representing the users of the multiusercommunication session inside the sphere, thereby providing each userwith a view of the other participants on the tropical island. See forexample ill which describes such a ‘Social VR’ use-case, albeit for animage-based instead of video-based backdrop.

The video is typically a 2D video, or a stereoscopic 3D video which isrecorded and intended to be viewed from a particular viewpoint. However,for a more immersive experience in both AR and VR, a viewer may desiresix degrees of freedom (6DoF). That is, when for example wearing ahead-mounted AR or VR display, the viewer may experience changes in theenvironment when moving his/her head in all directions, e.g., whenchanging head position forward/backward (surge), up/down (heave) andleft/right (sway) combined with changes in orientation through rotation.

It is known that volumetric 3D video may provide a viewer with a 6DoFexperience. Examples of volumetric 3D video formats, which are in thefollowing also simply referred to as volumetric video formats, are forexample described in [2], and may include Gabor holograms, light fields,point clouds and other video formats.

However, volumetric videos are not yet widely distributed, neither inthe domain of VR/AR nor in other domains, in part due to a limitedavailability of suitable source material (e.g., camera recordings and/ordepth registrations, e.g., by a depth sensor or laser) but also due totechnical considerations, for example since a volumetric video mayrepresent a large amount of data and may thereby require significantstorage, bandwidth and encoding and decoding capabilities.

For such and similar reasons, it may be desirable to use a combinationof 2D video or stereoscopic 3D video (henceforth jointly referred to as‘non-volumetric’ video) with one or more select instances of volumetricvideo. Such instances of volumetric video may represent 3D objects ofinterest. For example, in the aforementioned use-case of a multiusercommunication session, the avatars of participants may instead berepresented by a volumetric capture of each participant, e.g., as may beobtained using a 3D sensor camera such as the Microsoft Kinect.

A specific example of volumetric video may be a 3D point cloud video. Ingeneral, a point cloud may be defined as a set of data points in acoordinate system, and is typically used to measure and representphysical world surfaces. Usually, a 3D point cloud is defined in aCartesian coordinate system, e.g., with a X-, Y- and Z-coordinate. Atime-series of such 3D point clouds may also be simply referred to as a3D point cloud video. Such 3D point cloud videos may be in color, inthat points may be assigned a color attribute, e.g., a luminance and achrominance value.

Compression techniques may be applied to volumetric videos to reduce theamount of data to be stored, transmitted, etc. For example, for 3D pointclouds, so-called Point Cloud Compression (PCC) [3] may be used.However, PCC and similar techniques are currently not optimized forstreaming, e.g., from a server to a client device. Additionally, aclient device may now receive multiple compressed and encoded datastreams, namely a bitstream representing the non-volumetric video andseparately a bitstream representing the volumetric video. It may bechallenging for the client device to receive and decode multiplebitstreams simultaneously. For example, the decoding of a volumetricvideo stream may be more computationally complex, e.g., as it may haveto be performed in software instead of relying on hardware support,e.g., dedicated Graphics Processing Unit (GPU) support. Such hardwaresupport typically is available for decoding non-volumetric video. Arelated problem may be the relatively large bandwidth required totransmit both bitstreams.

REFERENCES

-   [1] M. J. Prins, S. Gunkel and O. Niamut, “TogetherVR: A Framework    for Photo-Realistic Shared Media Experiences in 360-Degree VR” in    International Broadcasting Conference, 2017.-   [2] Philip A. Chou, “Holograms are the Next Video” in Proceedings of    the 9th ACM Multimedia Systems Conference, 2018.-   [3] MPEG Point Cloud Compression (PCC group):    https://mpeg.chiariglione.org/standards/mpeg-i/point-cloud-compression

SUMMARY OF THE INVENTION

It would be advantageous to enable streaming a non-volumetric video anda volumetric video to a client device at a reduced bandwidth, forexample compared to the streaming of a separate non-volumetric videostream and a separate volumetric video stream. Additionally oralternatively, it would be advantageous to enable a client device todecode a non-volumetric video and a volumetric video at a reducedcomputational complexity, for example compared to the decoding of aseparate non-volumetric video stream and a separate volumetric videostream.

The following measures may be based on the consideration that encoding,streaming and decoding techniques for non-volumetric video may beconsidered mature and optimized, while such techniques for volumetricvideo may be still emerging. In addition, it is considered that avolumetric video may be typically displayed in front of a non-volumetricvideo, such as in the aforementioned ‘Social VR’ example in which avolumetric video recording of a user may be displayed in front of anon-volumetric video representing the ‘virtual backdrop’ of the scene.

In accordance with a first aspect of the invention, a processor systemis provided which may be configured for generating a composite videostream for transmission to a client device. The processor system maycomprise:

-   -   a network interface to a network;    -   an input interface for obtaining:        -   a non-volumetric video; and        -   a volumetric video, at least part of which is to be            displayed by the client device in front of the            non-volumetric video;    -   a processor which may be configured to:        -   determine a spatial subregion of the non-volumetric video            which is partially or entirely occluded when the volumetric            video is displayed by the client device in front of the            non-volumetric video; and        -   generate a composite video for the client device by, for            respective input frames of the non-volumetric video and the            volumetric video:            -   obtaining a non-volumetric representation of the                volumetric video which is generated using a conversion                technique which allows the volumetric video to be                reconstructed from the non-volumetric representation:            -   replacing data of the spatial subregion of the                non-volumetric video by data of the non-volumetric                representation of the volumetric video, thereby                obtaining an output frame of the composite video;    -   and via the network interface:        -   stream the composite video as a composite video stream, or            stream select spatial segments of the composite video            stream, to the client device; and        -   provide signaling data to the client device which is            indicative of the composite video stream containing the            non-volumetric representation of the volumetric video.

In accordance with a further aspect of the invention, acomputer-implemented method is provided which may generate a compositevideo stream for transmission to a client device. The method maycomprise:

-   -   obtaining:        -   a non-volumetric video; and        -   a volumetric video, at least part of which is to be            displayed by the client device in front of the            non-volumetric video;    -   determining a spatial subregion of the non-volumetric video        which is partially or entirely occluded when the volumetric        video is displayed by the client device in front of the        non-volumetric video;    -   generating a composite video for the client device by, for        respective input frames of the non-volumetric video and the        volumetric video:        -   obtaining a non-volumetric representation of the volumetric            video which is generated using a conversion technique which            allows the volumetric video to be reconstructed from the            non-volumetric representation;        -   replacing data of the spatial subregion of the            non-volumetric video by data of the non-volumetric            representation of the volumetric video, thereby obtaining an            output frame of the composite video;    -   streaming the composite video as a composite video stream, or        streaming select spatial segments of the composite video stream,        to the client device; and    -   providing signaling data to the client device which is        indicative of the composite video stream containing the        non-volumetric representation of the volumetric video.

In accordance with a further aspect of the invention, a processor systemis provided which may represent a client device configured to render avolumetric video in front of a non-volumetric video. The processorsystem may comprise:

-   -   a network interface to a network;    -   a processor which may be configured to, via the network        interface:        -   receive a composite video stream containing the            non-volumetric video, or select spatial segments of the            composite video stream;        -   receive signaling data which is indicative of the composite            video stream containing a non-volumetric representation of            the volumetric video in a spatial subregion of the            non-volumetric video;    -   wherein the processor may be configured to render the composite        video stream by, for a respective input frame of the composite        video stream:        -   decoding the composite video stream, or the select spatial            segments of the composite video stream;        -   on the basis of the signaling data, identifying the            non-volumetric representation of the volumetric video in the            input frame;        -   reconstructing the volumetric video from the non-volumetric            representation of the volumetric video using a            reconstruction technique; and        -   rendering the volumetric video in front of the            non-volumetric video so that the spatial subregion of the            non-volumetric video is partially or entirely occluded by            the volumetric video.

In accordance with a further aspect of the invention, acomputer-implemented method is provided which may render a volumetricvideo in front of a non-volumetric video. The method may comprise:

-   -   receiving a composite video stream containing the non-volumetric        video, or select spatial segments of the composite video stream;    -   receiving signaling data which is indicative of the composite        video stream containing a non-volumetric representation of the        volumetric video in a spatial subregion of the non-volumetric        video;    -   rendering the composite video stream by, for a respective input        frame of the composite video stream:        -   decoding the composite video stream, or the select spatial            segments of the composite video stream;        -   on the basis of the signaling data, identifying the            non-volumetric representation of the volumetric video in the            input frame;        -   reconstructing the volumetric video from the non-volumetric            representation of the volumetric video using a            reconstruction technique; and        -   rendering the volumetric video in front of the            non-volumetric video so that the spatial subregion of the            non-volumetric video is partially or entirely occluded by            the volumetric video.

In accordance with a further aspect of the invention, a transitory ornon-transitory computer-readable medium is provided comprising acomputer program. The computer program may comprise instructions forcausing a processor system to perform either or bothcomputer-implemented methods.

The above measures may involve generating a composite video stream for aclient device. The composite video stream may combine a volumetric videoand a non-volumetric video into one stream. The volumetric video may, atleast in part, be displayed by the client device in front of thenon-volumetric video. Here, the term ‘in front’ may refer to a relativedisplay of the volumetric video and the non-volumetric video whichcauses a part of the non-volumetric video to be partially or entirelyoccluded when the volumetric video and the non-volumetric video are bothdisplayed by the client device. For example, a volumetric videorecording of a user may be displayed in front of, and thereby fully orto a certain degree occlude part of, the non-volumetric videorepresenting the ‘virtual backdrop’ of a scene. Here, the term ‘to acertain degree’ may refer to an occlusion which locally reduces thevisibility of the underlying non-volumetric data, e.g., due totransparency of the volumetric video.

The above measures may further involve determining the spatial subregionof the non-volumetric video which is partially or entirely occluded whenthe volumetric video is displayed by the client device in front of thenon-volumetric video. For example, the spatial subregion may bedetermined based on signaling data received from the client device, orthe processor system generating the composite video stream mayinherently be aware of such information, e.g., by the processor systemitself prescribing the relative display positions of the non-volumetricvideo and the volumetric video to the client device, e.g., due to theprocessor system acting as an orchestrator.

The above measures may further involve generating a non-volumetricrepresentation of the volumetric video using a conversion techniquewhich allows the volumetric video to be reconstructed from thenon-volumetric representation. Such conversion techniques are known perse. For example, a 3D point cloud video may be converted to anon-volumetric (2D) video using a conversion technique known aspatch-based point cloud compression. Such type of conversions maygenerally allow the volumetric video to be reconstructed from itsnon-volumetric representation, either perfectly (e.g., losslessreconstruction) or imperfectly (e.g., lossy reconstruction), for exampleby applying a technique which is conceptually inverse to the conversiontechnique, the former being also referred to as a ‘reconstructiontechnique’. In some embodiments, the non-volumetric representation ofthe volumetric video may have been generated by another entity, e.g., acapture device, and the non-volumetric representation of the volumetricvideo may then be accessed by the processor system. Such generating oraccessing of the non-volumetric representation of the volumetric videomay be jointly referred to as ‘obtaining’ said non-volumetricrepresentation.

The non-volumetric representation of the volumetric video may then beinserted into the non-volumetric video, namely by replacing data in thepreviously identified spatial subregion of the non-volumetric video bydata of said non-volumetric representation of the volumetric video.Effectively, data of the non-volumetric representation may replace thedata of a part of the non-volumetric video which may not be visible to auser anyway as it will be entirely or partially occluded by thevolumetric video during display. The existing image data in this spatialsubregion may thus be replaced by some or all of data of thenon-volumetric representation without significant detriment to thesubsequent display of the non-volumetric video. In some embodiments,such replacement may comprise inserting of the data of thenon-volumetric representation of the volumetric video in the spatialsubregion of the non-volumetric video. Such insertion may involvereplacing the existing image data of the non-volumetric video in thespatial subregion by the data of said non-volumetric representation, forexample on a pixel-by-pixel, block-by-block or segment-by-segment basis.An example of a replacement on a segment-by-segment basis is areplacement on a tile-by-tile basis. In other embodiments, the data ofthe non-volumetric in the spatial subregion may be removed, and the dataof the non-volumetric representation of the volumetric video may beadded elsewhere to the non-volumetric video.

If the non-volumetric representation of the volumetric video is insertedinto the spatial subregion of the non-volumetric video. e.g., as‘inserted data’, this inserted data as such may not be visible duringthe subsequent display of the non-volumetric video as the spatialsubregion of the non-volumetric video may be entirely or partiallyoccluded by the volumetric video which is reconstructed from theinserted data.

In general, the data of the non-volumetric representation of thevolumetric video may not be recognizable by the user as representing thevolumetric video, but may be inserted as if it were image data.

Alternatively, if the video format of the non-volumetric video, or thevideo format of the resulting composite video or composite video stream,supports layers or conceptually similar techniques, the data of thenon-volumetric representation may be inserted as a layer overlaying theoriginal image data in the spatial subregion.

By performing the insertion, e.g., on a frame-by-frame basis, acomposite video may be obtained, which may be considered anon-volumetric composite video as it contains the non-volumetric videoand the non-volumetric representation of the volumetric video, and whichmay be streamed to the client device in the form of a composite videostream. For that purpose, the composite video may be encoded to obtainthe composite video stream. Alternatively, the non-volumetric video andthe non-volumetric representation of the volumetric video may beseparately encoded, and the composite video stream may be generated bycombining both encodings, e.g., on a segment-by-segment basis in case ofso-called spatially segmented encodings.

The composite video stream may then be streamed to the client device,either in its entirety or as select spatial segments, e.g., only thosewhich are visible to a user of the client device given the users currentfield of view (‘viewport’), e.g., in a VR environment. The latterconcept is also known in VR as ‘Viewport-Adaptive Streaming’ (VAS). Aspecific example of VAS is ‘tiled streaming’. In general, the term‘select’ when referring to the spatial segments may refer to ‘one ormore’ of the spatial segments.

Additionally, signaling data may be provided to the client device whichmay be indicative of the composite video stream containing thenon-volumetric representation of the volumetric video. For example, thesignaling data may identify the fact that the composite video streamcontains data of the non-volumetric representation of the volumetricvideo, and if the data is specifically inserted into the spatialsub-region, identify the spatial subregion itself and/or the usedconversion technique.

Compared to a client device having to receive at least two separatevideo streams, e.g., a non-volumetric video stream and a volumetricvideo stream, the above measures may provide one or more advantages tothe client device. For example, the client device may only have todecode the composite video stream and thereby fewer video streams. Thismay provide compatibility with client devices having only one hardwaredecoder. Moreover, by converting the volumetric video to anon-volumetric representation thereof, the volumetric video may beencoded, transmitted and decoded as if it were a ‘conventional’non-volumetric video, e.g., a 2D video. The encoding, transmission anddecoding, or in general a used video workflow or video pipeline, maythus not have to be modified to support volumetric video. Rather, it maysuffice for the encoding side to be able to convert the volumetric videoto the non-volumetric representation before encoding, and for the clientdevice to be able to reconstruct the volumetric video from thenon-volumetric representation after decoding. Moreover, the compositevideo stream may omit the parts of the non-volumetric video which areoccluded by the volumetric video, and may thus be smaller in size than anon-volumetric video stream and an additional volumetric video stream(as the former contains image data which is or will be occluded by thelatter when displayed, e.g., in a VR environment). This may reduce thecomputational complexity of decoding and bandwidth and storagerequirements for receiving and buffering. Moreover, if there are severalvolumetric videos which are inserted centrally by an entity, such as theaforementioned processor system, the client device would only need toreceive and decode the composite video stream instead of having toreceive and decode the volumetric video streams separately, possiblyfrom different entities.

The bandwidth advantages may also apply to the (access) network by whichthe composite video stream is transmitted to the client device.

In this respect, it is noted that, in general, not all of the volumetricvideo may be inserted into the non-volumetric video, but rather only apart of the volumetric video, which may be a substantial part. Anyreferences to ‘conversion’, ‘insertion’, ‘render’, etc. of thevolumetric video is to be understood as including said actions appliedto only a (substantial) part of the volumetric video. For example, if avolumetric video contains a volumetric recording of a user of amultiuser communication session and his/her immediate surroundings, onlythe data representing the user may be inserted into the non-volumetricvideo while omitting inserting the data of his/her surroundings.

In general, the volumetric video may be considered a ‘foreground video’and the non-volumetric video may be considered a ‘background video’ asthe volumetric video may be displayed in front of the non-volumetricvideo by the client device. However, the terms ‘foreground’ and‘background’ are not technically limiting with respect to the (semantic)content of the respective videos, but rather refer to a display order inthat the foreground video is to be displayed ‘in front of’ thebackground video. For example, a background video may typically containthe background of a scene, but a background video may also comprise oneor more foreground objects. As a specific example, in a VR multiusercommunication setting, the background video stream may provide abackground of the conference room and a table in the foreground, whereasthe foreground video may be a real-time volumetric video of a user thatparticipates in the VR multiuser communication session. Likewise, thevolumetric ‘foreground’ video may itself be partially or entirelyoccluded by another foreground object, such as another volumetric video,a 3D graphics object, etc. For example, the real-time volumetric videoof a user may be partially occluded by a table, e.g., as represented bya 3D graphics object, to convey the impression that the user is seatedbehind the table.

In the above and following, the term ‘rendering’ may refer to anoperation which may process input data to obtain displayable data. Insome embodiments, the input data may not represent displayable data. Inother embodiments, the input data may represent displayable data per se,but the rendering may provide another type of displayable data. Suchrendering may include, but is not limited to, Central Processing Unit(CPU)-based rendering and Graphics Processing Unit (GPU)-basedrendering.

In general, the foreground video may be more dynamic than the backgroundvideo. Hence, the foreground video may be obtained in real-time, whereasthe background video may be pre-recorded. However, this is not alimitation, as the background video may also be obtained in real-time.e.g., live recorded, and/or the foreground video may alternatively bepre-recorded. Both or either video may already be obtained as a videostream, e.g., in a streamable and encoded form.

The following embodiments are described with reference to thecomputer-implemented method and the processor system for generating thecomposite video stream, but may denote corresponding embodiments of thecomputer-implemented method and the processor system for rendering thecomposite video stream.

In an embodiment, the non-volumetric video may be obtained as, orconverted into, a spatially segmented encoding comprising independentlydecodable segments, and the processor may be configured to:

-   -   determine the spatial subregion as a set of segments of the        non-volumetric video which are occluded when the volumetric        video is displayed by the client device in front of the        non-volumetric video;    -   encode the non-volumetric representation of the volumetric video        as one or more independently decodable segments; and    -   remove the set of segments from the spatially segmented encoding        of the non-volumetric video, and add the one or more segments to        the spatially segmented encoding of the non-volumetric video, to        obtain a spatially segmented encoding of the output frame of the        composite video stream.

Spatially segmented encoding techniques are known per se. For example,as spatial segments, so-called tiles' may be used which may subdivide avideo frame into logically separate rectangular parts that may bedecoded independently when decoding a given frame. The tiles may then berequested and streamed individually by a client device on the basis of aso-called manifest. Example of ‘tiled streaming’ techniques aredescribed in [4] and [5] (see ‘Further references’), and may involvedescribing the relationship between tiles in the form of a SpatialRelationship Description (SRD) or similar data, and including said datain a manifest, such as an MPD (Media Presentation Description). Tilesmay then be requested individually by the client device on the basis ofthe manifest, for example those in a current field of view.

If the non-volumetric video is obtained as, or converted into, aspatially segmented encoding, the subsequent insertion of the data ofthe non-volumetric representation of the volumetric video may be lesscomputationally complex. Namely, specific spatial segments of thenon-volumetric video may be identified which may be partially orentirely occluded by the volumetric video. The spatial segments whichare entirely occluded may then be removed from the spatially segmentedencoding of the non-volumetric video, and the spatial segmentsrepresenting the non-volumetric representation of the volumetric videomay be added instead.

It is noted that in some embodiments, this may represent a replacementof segments in the spatially segmented encoding of the non-volumetricvideo. However, in other embodiments, the number of segments which maybe added to the spatially segmented encoding of the non-volumetric videomay exceed the number of segments which may be removed from saidspatially segmented encoding.

For spatial segments which are only partially occluded, the spatialsegment(s) may be decoded, the data of the non-volumetric representationof the volumetric video may be inserted, and the spatial segment(s) maybe re-encoded. As such, in some embodiments, only those segments may beremoved which are entirely occluded. In other embodiments, only thosesegments which are at least occluded above a certain degree may beremoved, for example 50% of the segment's area.

From the perspective of the processor system generating the compositevideo stream, the use of a spatially segmented encoding may reduce thecomputational complexity of the insertion, particularly if thenon-volumetric video is already available in such form. Namely, in thiscase, it may not be needed to decode the entire non-volumetric video,insert the non-volumetric representation of the volumetric video intothe non-volumetric video, and encode the resulting composite video.Rather, it may suffice to encode the non-volumetric representation ofthe volumetric video as one or more spatial segments, and include thesesegments in the spatially segmented encoding of the non-volumetric videowhile omitting segments which are occluded.

Since the non-volumetric video may be relatively large, for exampleproviding a 180-degree or 380-degree high-resolution view of a scene,typically only a small part of the segments may have to be processed.This may reduce the computational complexity of the processing, but alsothe latency caused by the processing. The latter may be particularlyrelevant if the volumetric video is used for communication purposes, forexample when representing a real-time volumetric recording of a user, aslatency may disturb the communication between users.

In an embodiment, the processor may be configured to generate thesignaling data to identify the set of segments as containing thenon-volumetric representation of the volumetric video. For example, thesignaling data may contain identifiers of each of said segments. Anotherexample is that the signaling data may contain, for each segment, anidentifier denoting the media type of the respective segment, e.g.,non-volumetric video, or a non-volumetric representation of a volumetricvideo. The signaling data may thus identify each segment as eithercontaining part of the non-volumetric video or part of thenon-volumetric representation of the volumetric video. The client devicemay thereby on the basis of the signaling data identify which segmentsare to be used as input for the reconstruction of the volumetric video.

In an embodiment, the processor may be configured to include thesignaling data in the composite video stream, for example as aSupplemental Enhancement Information (SEI) message. By including thesignaling data in the composite video stream, there may be no need for aseparate signaling channel to the client device.

In an embodiment, the processor may be configured to generate thesignaling data by generating or modifying a manifest associated with thecomposite video stream to identify the set of segments of the compositevideo stream which contain the non-volumetric representation of thevolumetric video.

A non-limiting example of a manifest within the context of MPEG-DASH isan MPD (Media Presentation Description). Other types of manifests areknown as well, and may, within the context of spatially segmentedstreaming, identify the spatial segments available for streaming andtheir location (e.g., URL, filename, port number, etc.) at which theymay be retrieved. Such a manifest may contain additional metadata, andmay therefore be generated or modified to identify the set of segmentswhich contain the non-volumetric representation of the volumetric video.Effectively, the manifest may comprise or represent the signaling dataas described elsewhere.

In an embodiment, the client device may be configured to render thecomposite video stream in a Virtual Reality (VR) or Augmented Reality(AR) environment and to render the VR/AR environment from a viewingposition of a user, and the processor may be configured to:

-   -   determine the viewing position of the user, for example by        receiving data indicative of the viewing position from the        client device; and    -   determine the spatial subregion by determining which spatial        subregion of the volumetric video is partially or entirely        occluded by the volumetric video when the volumetric video is        displayed in front of the non-volumetric video in the VR/AR        environment and rendered by the client device from the viewing        position.

The composite video stream may be generated taking into account theviewing position of a user of the client device in a VR/AR environment,for which user the composite video stream is generated. Here andelsewhere, it is to be understood that “of the user” may technicallycorrespond to the provision of a user-adjustable parameter and theprovision of a mechanism for the user to adjust said parameter.

The viewing position may be characterized in various ways, e.g., as a 2Dor 3D position in the VR/AR environment. If the volumetric video is notto be ‘glued’ onto the non-volumetric video but rather to be representedin the VR environment as a separate object which is to be placed infront of the object representing the non-volumetric video (e.g., asurrounding sphere), the viewing position may determine which part ofthe non-volumetric video may be occluded by the volumetric video, andthereby the spatial subregion into which data of the non-volumetricrepresentation of the volumetric video may be inserted. By taking intoaccount the viewing position, it may thus be avoided that thenon-volumetric representation of the volumetric video is inserted into apart of the non-volumetric video which would otherwise be visible to theuser from his/her viewing position, e.g., not occluded by the volumetricvideo.

It is noted that the viewing position of the user may be determined invarious ways, for example by receiving data indicative of the viewingposition from the client device. Such data may be received only once,e.g., before starting to generate the composite video stream, orregularly, e.g., reflecting a current viewing position of the user.However, the viewing position may also be estimated or predetermined,e.g., corresponding to one of a limited number of viewing positions, orsimply fixed.

In many cases, the client device may only render a VR/AR environment ina particular field of view (also referred to as ‘viewport’) which may besmaller than the overall field of view provided by the VR/ARenvironment, the latter being typically a 360-degree field of view. Asparts of the VR/AR environment may not be visible to the user at a givenmoment in time, this may be taken into account when inserting data ofthe non-volumetric representation of the volumetric video into thenon-volumetric video. In an embodiment, inserting the data of thenon-volumetric representation of the volumetric video into the spatialsubregion of the non-volumetric video based on the field of view maycomprise only inserting the non-volumetric representation of thevolumetric video into the spatial subregion of the non-volumetric videoif said spatial subregion is in the field of view of the user, or withina vicinity of the field of view.

It is noted that the client device may use ‘tiled streaming’ or similarviewport-adaptive streaming techniques to only selectively stream one ormore spatial segments which are within the field of view (and/or withina narrow ‘sideband’ around the field of view) of the user. The requestedsegments may therefore be indicative of the current field of view of theclient device.

In an embodiment, the processor system generating the composite videostream may be a network node of a telecommunication network, such as anedge node, e.g., in a 5G or next generation telecommunication network.Such edge nodes of 5G or next generation telecommunication networks mayhave a (very) low delay to client devices and may be well suited forinserting the non-volumetric representation of the volumetric video intoa dynamically changing spatial subregion, for example one which isdependent on a current viewing position of a user of the client device.

The following embodiments are described with reference to thecomputer-implemented method and the processor system for rendering thecomposite video stream, but may denote corresponding embodiments of thecomputer-implemented method and the processor system for generating thecomposite video stream.

In an embodiment, the composite video stream may be received as aspatially segmented encoding which comprises independently decodablesegments, and the processor may be configured to identify thenon-volumetric representation of the volumetric video in the compositevideo stream based on the signaling data identifying a set of segmentsof the spatially segmented encoding. As also described earlier, theclient device may thereby on the basis of the signaling data identifywhich segments are to be used as input to the reconstruction of thevolumetric video.

In an embodiment, the signaling data may be received as part of amanifest associated with the composite video stream. The manifest mayidentify the set of segments containing the non-volumetricrepresentation of the volumetric video.

In an embodiment, the processor may be configured to render thecomposite video stream in a Virtual Reality (VR) or Augmented Reality(AR) environment, and render the VR/AR environment from a viewingposition of a user. For example, the non-volumetric video may be usedas, or displayed onto, a background object in the VR/AR environment,while the volumetric video which may be reconstructed on the basis ofthe data contained in the composite video stream may be displayed as aforeground object in the VR/AR environment in front of the background.

The following embodiments are described with reference to thecomputer-implemented method and the processor system for rendering thecomposite video stream, and with reference to the computer-implementedmethod and the processor system for generating the composite videostream.

In an embodiment, the volumetric video may be a 3D point cloud, and theconversion technique by which the non-volumetric representation of thevolumetric video is generated may be a point cloud compressiontechnique, for example a patch-based point cloud compression technique.Point cloud compression techniques such as patch-based point cloudcompression techniques are well suited for converting 3D point cloudsinto a non-volumetric form, e.g., into 2D image data. In an embodiment,the volumetric video may be a light field, and the non-volumetricrepresentation of the light field may be a grid of 2D videos from whichthe light field may be reconstructed.

In an embodiment, the non-volumetric video is a panoramic oromnidirectional video. The non-volumetric video may, for example,incorporate a map projection, such as an equirectangular projection orcube-map projection.

The following aspects of the invention and embodiments relate tosignaling data, but may denote corresponding embodiments of thecomputer-implemented method(s) and processor system(s) generating and/orusing the signaling data.

In a further aspect of the invention, a transitory or non-transitorycomputer-readable medium may comprise signaling data which may beassociated with a composite video stream containing a non-volumetricvideo and which may be indicative of the composite video streamcontaining a non-volumetric representation of a volumetric video in aspatial subregion of the non-volumetric video.

In an embodiment, the composite stream may be a spatially segmentedencoding which comprises independently decodable segments, and thesignaling data may identify a set of segments of the spatially segmentedencoding. As also described earlier, the client device may thereby onthe basis of the signaling data identify which segments are to be usedas input to the reconstruction of the volumetric video.

In an embodiment, the transitory or non-transitory computer-readablemedium may comprise a manifest associated with the composite videostream, and the manifest may identify the set of segments of thespatially segmented encoding which contain the non-volumetricrepresentation of the volumetric video. Effectively, the manifest maycomprise or represent the signaling data as described elsewhere.

In accordance with an abstract of the present invention, a processorsystem and computer-implemented method may be provided for generating acomposite video stream which may include a non-volumetric video and avolumetric video, at least part of which is to be displayed in front ofthe non-volumetric video. The volumetric video may be included in thecomposite video stream in the form of a non-volumetric representation ofthe volumetric video, for example inserted into a spatial subregion ofthe non-volumetric video which may be occluded by the volumetric videoduring display. The encoding, transmission and decoding may thus nothave to be modified to support volumetric video. Signaling data may beprovided which may be indicative of the composite video streamcontaining the non-volumetric representation of the volumetric video. Aprocessor system and computer-implemented method may be provided forrendering the composite video stream using the signaling data.

It will be appreciated by those skilled in the art that two or more ofthe above-mentioned embodiments, implementations, and/or aspects of theinvention may be combined in anyway deemed useful.

Modifications and variations of the method, the processor system(s), themetadata and/or the computer program, which correspond to themodifications and variations described for another one of said entities,can be carried out by a person skilled in the art on the basis of thepresent description.

FURTHER REFERENCES

-   [4] Ochi, Daisuke, et al. “Live streaming system for omnidirectional    video”, Virtual Reality (VR), 2015 IEEE.-   [5] K. Misra, A. Segall. M. Horowitz, S. Xu and A. Fuldseth, “An    Overview of Tiles in HEVC” IEEE Journal of Selected Topics in Signal    Processing, 2013.-   [6] Y. Sanchez, R. Globisch, T. Schierl and T. Wiegand, “Low    Complexity Cloud-video-Mixing Using HEVC” CCNC, no. 11. pp. 213-218,    2014.-   [7] M. S. A. H. Peter Amon, “Compressed Domain Stitching of HEVC    Streams for Video Conferencing Applications” in International Packet    Video Workshop, Munich, 2012.-   [8] S. Schwarz et al., “Emerging MPEG Standards for Point Cloud    Compression”, IEEE Journal on Emerging and Selected Topics in    Circuits and Systems. December 2018. doi:    10.1109/JETCAS.2018.2885981.-   [9] C. Perra and D. Giusto, “Raw light field image compression of    sliced lenslet array”, 2017 IEEE International Symposium on    Broadband Multimedia Systems and Broadcasting (BMSB), pp. 1-5, 2017.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter. Inthe drawings,

FIG. 1A provides an illustration of a non-volumetric video of a scene;

FIG. 1B provides an illustration of a volumetric video of an object;

FIG. 1C shows a composition of the videos of FIGS. 1A and 1B, which mayfor example be generated by a client device receiving the non-volumetricvideo and the volumetric video, in which the volumetric video of theobject is displayed in front of the non-volumetric video, therebyoccluding a part of the non-volumetric video;

FIG. 2A illustrates a first part of generating of a composite videowhich contains a non-volumetric representation of the volumetric videoin a spatial subregion of the non-volumetric video, namely thenon-volumetric video being obtained as a spatially segmented encodingcomprising independently decodable segments;

FIG. 2B shows spatial segments of the non-volumetric video which are atleast partially occluded when the volumetric video is displayed in frontof the non-volumetric video, and which may be replaced by, or removedwhile adding further segments, containing the non-volumetricrepresentation of the volumetric video;

FIG. 3 shows steps of encoding a composite video stream which comprisesa non-volumetric video and a non-volumetric representation of avolumetric video, wherein the non-volumetric representation may beinserted into a spatial subregion of the non-volumetric video, andwherein signaling data may be provided which may be indicative of thecomposite video stream containing the non-volumetric representation;

FIG. 4 shows steps of decoding and rendering a composite stream, whichmay for example be generated by the steps of FIG. 3 , using signalingdata;

FIG. 5 illustrates data communication between a processor systemconfigured for generating a composite video stream and a processorsystem configured for decoding and rendering the composite video streamin a VR environment;

FIG. 6 shows a processor system which may be configured for generating acomposite video stream for transmission to a client device;

FIG. 7 shows a processor system which may represent the client deviceand which may be configured to decode and render the composite videostream;

FIG. 8 shows a method of generating the composite video stream;

FIG. 9 shows a method of rendering the composite stream;

FIG. 10 shows a computer-readable medium comprising non-transitory data;

FIG. 11 shows an exemplary data processing system.

It should be noted that items which have the same reference numbers indifferent figures, have the same structural features and the samefunctions, or are the same signals. Where the function and/or structureof such an item has been explained, there is no necessity for repeatedexplanation thereof in the detailed description.

LIST OF REFERENCE AND ABBREVIATIONS

The following list of references and abbreviations is provided forfacilitating the interpretation of the drawings and shall not beconstrued as limiting the claims.

-   -   100 non-volumetric background video    -   110 spatially segmented encoding of non-volumetric video    -   120 spatial segments occluded by volumetric video    -   200 volumetric foreground video    -   300 3D volumetric video    -   305 3D to 2D mapping    -   310 2D representation of 3D volumetric video    -   315 tile and encode    -   320 tiled 2D representation of 3D volumetric video    -   330 2D panoramic video    -   335 tile and encode    -   340 tiled 2D panoramic video    -   345 user viewport    -   350 occlusion detection    -   355 tile replacement    -   360 generate signaling    -   365 composite video with signaling    -   400 composite video with signaling    -   410 signaling extraction    -   420 tile decoder    -   430 2D representation of 3D volumetric video    -   440 2D to 3D mapping    -   450 3D volumetric video    -   460 2D panoramic video    -   470 rendering of 2D panoramic video and 3D volumetric video    -   500 processor system for generating composite video stream    -   510 composite video stream    -   520 signaling data    -   530 network    -   540 processor system for rendering composite video stream    -   550 display data    -   560 head mounted display    -   600 processor system for generating composite video stream    -   610 network interface    -   612 network data communication    -   620 input interface    -   625 data storage    -   630 processor    -   700 processor system for rendering composite video stream    -   710 network interface    -   712 network data communication    -   720 processor    -   730 output interface    -   732 display data    -   735 display    -   800 method of generating composite video stream    -   810 obtaining non-volumetric video and volumetric video    -   820 determining spatial subregion in non-volumetric video    -   830 generating composite video stream    -   832 generating non-volumetric representation of volumetric video    -   834 replacing data of spatial subregion of non-volumetric video        by data of non-volumetric representation of volumetric video    -   840 streaming composite video stream    -   845 providing signaling data    -   850 method of rendering composite video stream    -   860 receiving composite video stream    -   865 receiving signaling data    -   870 rendering composite video stream    -   872 decoding composite video stream    -   874 identifying non-volumetric representation of volumetric        video    -   876 reconstructing volumetric video    -   878 rendering volumetric video in front of non-volumetric video    -   900 computer readable medium    -   910 non-transitory data    -   1000 exemplary data processing system    -   1002 processor    -   1004 memory element    -   1006 system bus    -   1008 local memory    -   1010 bulk storage device    -   1012 input device    -   1014 output device    -   1016 network adapter    -   1018 application

1000 exemplary data processing system 1002 processor 1004 memory element1006 system bus 1008 local memory 1010 bulk storage device 1012 inputdevice 1014 output device 1016 network adapter 1018 application

DETAILED DESCRIPTION OF EMBODIMENTS

The following embodiments relate to the generating and rendering of acomposite video stream. Some embodiments are described within thecontext of spatially segmented streaming, e.g., ‘tiled streaming’ [4],[5], which may be used for the transmission of panoramic oromnidirectional videos, such as 360-degree videos. However, thetechniques described in this specification may also be applied to anyother type of streaming, including non-spatially segmented (non-tiled)streaming.

Some embodiments are described within the context of rendering thecomposite video stream in VR, such as a ‘Social VR’ virtual environmentwhere a number of users may participate in a teleconference using HeadMounted Displays (HMDs) and cameras. However, the techniques describedin this specification may also be applied in all other applications inwhich a volumetric video is to be displayed in front of a non-volumetricvideo and in which the volumetric video then covers (‘occludes’) a partof the non-volumetric video. A non-limiting example is the insertion ofa volumetric video of an animated channel logo into a non-volumetrictelevision program.

Some embodiments are described in which the non-volumetric video is a 2Dvideo, while the volumetric video is a 3D video. It will be appreciated,however, that the techniques described in this specification may also beapplied to non-volumetric 3D video, namely to stereoscopic 3D video.Also, the volumetric video may take different forms, includinghigher-dimensional video such as a 4D or 5D time-varying light field.

It is further noted that in the following, any reference to a ‘videostream’ may refer to a data representation of a video which is suitablefor being streamed, e.g., using known streaming techniques. Furthermore,a reference to a ‘video’ may include a video stream but also a datarepresentation of the video which is not (yet) suitable for beingstreamed or at least conventionally not intended for streaming. In theFigures, video (streams) may be schematically represented by a singlevideo frame.

FIGS. 1A-1C illustrate the combined display of a volumetric video and anon-volumetric video. Here, FIG. 1A shows a non-volumetric video 100which may represent a scene while FIG. 1B shows a volumetric video 200which may represent an object which is to be shown in the scene. Thevolumetric video 200 may be a 3D video which is, for sake ofillustration, shown in the form of a cubic image volume 200 whileschematically indicating its semantic contents, e.g., a skier. As shownin FIG. 1C, both videos may then be jointly displayed by rendering thevolumetric video 200 as a foreground object over the non-volumetricvideo 100 as a background.

It will be appreciated that while FIG. 1C shows the entire image volumeof the volumetric video 200 being rendered over the non-volumetric video100, the rendering may also be such that only a part of the image volume200 is rendered, e.g., only the skier and not the skier's immediatesurroundings within the cubic image volume. For example, if thevolumetric video 200 is a volumetric video comprised of voxels, therendering may omit rendering the surroundings of the skier on the basisof voxels surrounding the skier having been previously assigned a 100%transparency or 0% opacity. In another example in which the volumetricvideo 200 is a 3D point cloud, only the point cloud's points may berendered while the non-volumetric video 100 in the background may remainvisible in some spaces between the rendered points.

With continued reference to FIGS. 1A-1C, a composite video may begenerated which combines the non-volumetric video 100 and the volumetricvideo 200 into a single video, and if encoded as a stream, into a singlevideo stream. This may generally involve converting the volumetric videointo a non-volumetric representation of the volumetric video, e.g.,using techniques such as [8] in case the volumetric video is a 3D pointcloud, and by identifying a spatial subregion in the non-volumetricvideo which is occluded by the volumetric video when the volumetricvideo is displayed in front of the non-volumetric video. The data of thenon-volumetric video in the spatial subregion may then be replaced bydata of the non-volumetric representation of the volumetric video. Thismay involve inserting the non-volumetric representation of thevolumetric video into the spatial subregion in the non-volumetric video.In the specific case of the non-volumetric video being available as aspatially segmented encoding, such replacing may be carried out asdescribed with reference to FIGS. 2A and 2B.

FIGS. 2A and 2B illustrates the generating of a composite video for anon-volumetric video which is obtained as, or converted into, aspatially segmented encoding 110 of the non-volumetric video. Thespatially segmented encoding 110 may comprise so-called independentlydecodable spatial segments. For example, as spatial segments, so-called‘tiles’ [4], [5] may be used which may subdivide a video frame intologically separate rectangular parts that may be decoded independentlywhen decoding a given frame. The HEVC [5] standard defines the tileconfiguration for the entire frame as a homogenous regular grid, as alsodepicted by FIGS. 2A and 2B. The following may refer to spatial segmentsas ‘tiles’, with a reference to ‘tiles’ denoting spatial segments ingeneral, not only tiles as defined by the HEVC standard, unlessotherwise noted.

As previously indicated in FIG. 1C, the volumetric video 200 may occludea spatial subregion of the non-volumetric video when the volumetricvideo is displayed in front of the non-volumetric video. In case thenon-volumetric video is available as a spatially segmented encoding 110,the spatial subregion may be represented as a set of spatial segments120 which are occluded by the volumetric video 200 during display. Thespatial segments 120 may then be replaced by data of the non-volumetricrepresentation of the volumetric video, for example by removing the setof spatial segments 120 from the spatially segmented encoding 110 of thenon-volumetric video, and by encoding the non-volumetric representationof the volumetric video as one or more spatial segments. A compositevideo stream may then be generated to comprise the ‘remaining’ segmentsof the spatially segmented encoding 110 to which the one or more spatialsegments of the non-volumetric representation of the volumetric videomay be added. In an alternative embodiment, instead of removing the setof spatial segments 120 from the spatially segmented encoding 110, thespatially segmented encoding 110 may be directly generated without theset of spatial segments 120.

It will be appreciated that in some embodiments, only a part of aspatial segment may be occluded by the volumetric video 200. In such acase, it may not be desirable to omit the entire spatial segment, andinstead, the spatial segment may remain in the composite video stream.Accordingly, only spatial segments may be removed which are entirely, toat least partially, occluded by the volumetric video. Here, ‘partially’may refer to only a spatial part of the image data in the spatialsegment being occluded, and/or to the occlusion reducing the visibilityof the underlying non-volumetric data to a certain degree, e.g., due totransparency of the volumetric video. Alternatively, all spatialsegments which are at least in part occluded may be omitted from thecomposite video stream. In general, a low-resolution representation ofthe non-volumetric video may substitute for the omitted parts of thenon-volumetric video. Such low-resolution representations are also knownas ‘fallback layers’.

It is noted that the generating of the composite video may involverewriting the bitstream of the spatially segmented encoding 110 of thenon-volumetric video, e.g., in a manner as described in [6] and [7].Such rewriting of the bitstream may, for example, comprise changingparameters in the bitstream, e.g. high-level syntax parameters, such astile locations and dimensions in the Picture Parameter Set (PPS).

It will be appreciated that the segments representing the non-volumetricrepresentation of the volumetric video may be encoded at a higherbitrate than the bitrate at which the segments of the non-volumetricvideo were or are encoded, or in general, at a higher quality level. Forexample, suitable values of quantization parameters (QP) may be selectedto improve the quality. For example, in HEVC, QP values may range from0-51 with the highest quality being 0. Such increase of quality may forexample be specific to segments representing volumetric video containingimage data of users as such image data may be watched with moreattention.

FIGS. 3 and 4 show an encoding and subsequent decoding and rendering ofa composite video stream, which is in this specific example generated asa tiled composite video stream as also described with reference to FIGS.2A and 2B. In this example, the non-volumetric video is a 2D panoramicvideo and the volumetric video is a 3D volumetric video, or simply ‘3Dvideo’. Although the steps illustrated by FIGS. 3 and 4 are described ina particular order, the steps may be performed in any suitable order,e.g., consecutively, simultaneously, or a combination thereof, subjectto, where applicable, a particular order being necessitated, e.g., byinput/output relations.

FIG. 3 illustrates the encoding of a composite video stream. Here, a 3Dvolumetric video 300 may be obtained, which may be converted into anon-volumetric (2D) representation of the 3D video using a conversiontechnique which is indicated in FIG. 3 as a ‘3D to 2D mapping’ 305,thereby obtaining a 2D representation 310 of the 3D video. The 2Drepresentation 310 may be tiled and encoded 315, thereby obtaining atiled and encoded 2D representation 320 of the 3D video. Furthermore, a2D panoramic video 330 may be obtained, which may be tiled and encoded335, thereby obtaining a tiled 2D video 340. Occlusion detection 350 maybe performed, which may take into account the user's viewport 345, asalso described elsewhere in this specification. Occluded tiles of thetiled 2D video 340 may be replaced 355 by tiles of the tiled 2Drepresentation 320 of the 3D video, thereby obtaining a composite videostream. Signaling data may be generated 360 to indicate that thecomposite video stream comprises tiles of the tiled 2D representation320. The signaling data may be included in the composite video stream,yielding a composite video stream with signaling data 365. For example,Supplemental Enhancement Information (SEI) messages may be included inthe bitstream to indicate the tile type, e.g., non-volumetric video or anon-volumetric representation of a volumetric video.

FIG. 4 shows a decoding and rendering of the composite video stream ofFIG. 3 , which is in FIG. 4 indicated by reference numeral 400. Thesignaling data may be extracted 410 from the composite video stream.Using the signaling data, tiles of the composite video stream may bedecoded 420 to obtain data of the 2D representation 430 of the 3D video,and data of the 2D video 460. The former may be used as input to areconstruction technique which is indicated in FIG. 4 as a ‘2D to 3Dmapping’ 440, thereby obtaining a reconstruction of the 3D volumetricvideo 450. Both the 2D video 460 and the reconstructed 3D volumetricvideo 450 may then be rendered, e.g., in a VR-based virtual environmentwhich may be displayed to a user using an HMD.

FIG. 5 illustrates data communication between a processor system 500configured for generating a composite video stream and a processorsystem 540 configured for rendering the composite video stream, being inthis example a rendering in a Virtual Reality (VR) environment. Ingeneral, the processor system 500 may represent an entity generating thecomposite video stream 510 as also described elsewhere, e.g., withreference to FIGS. 1A-4 and FIG. 6 . The processor system 500 may alsobe referred to as ‘encoder system’ and may be embodied by, for example,a server or a distributed system of servers. The processor system 540may represent an entity decoding and rendering the composite videostream 510 as also described elsewhere, e.g., with reference to FIGS.1A-4 and FIG. 7 . The processor system 540 may also be referred to as‘receiver system’ and may be embodied by a client device, such as forexample a computer, console, smartphone or tablet device.

The processor system 500, which may for example be a cloud-based server,may generate and stream the composite video stream 510 to the processorsystem 540, e.g., via a network 530 such as the Internet and/or anaccess network and/or core network of a telecommunication network. Uponreceiving the composite video stream 510, the processor system 540 mayestablish a visual rendering of a VR environment in which thenon-volumetric video and the reconstructed volumetric video may bedisplayed. The processor system 540 may then output rendered image dataas display data 550 to an HMD 560 worn by a user. Before or during thestreaming of the composite video stream 510, the processor system 500may provide signaling data 520 to the processor system 540 which mayindicate that the composite video stream 510 contains the non-volumetricrepresentation of the volumetric video. This may effectively signal theprocessor system 540 that the volumetric video may be reconstructed bythe processor system 540 from data contained in the composite videostream 510.

Conversion and Reconstruction

To obtain a conversion from a volumetric video to a non-volumetricrepresentation of the volumetric video, and a reconstruction of thevolumetric video from its non-volumetric representation, varioustechniques may be used. For example, for 3D point clouds, the techniquesdescribed in [8] may be used which may involve segmenting the 3D pointcloud on the basis of a given feature, for example a planar feature. Thecolor information of the 3D point cloud may then be ‘unwrapped’ toobtain 2D image data, and the depth information may be extracted as adepth map which may also be represented as 2D image data. Both 2D imagedata parts may be included the composite video. At the receiver, e.g.,at a client device, the 3D point cloud may then be reconstructed usingincluded the depth map and the color information.

Another example of a volumetric video which may be included in thecomposite video is a light field, which may be considered a form ofvolumetric video which describes incoming light from all directions at agiven sample point. A light field may be represented by a 2D rectangulargrid of 2D images. Similarly, a time-varying light field may berepresented by a 2D rectangular grid of 2D videos [9]. Such a 2D grid of2D videos may thus be considered as a 2D video-based representation of alight field. The formatting of the light field as a 2D rectangular gridof 2D videos may be considered a conversion of the light field into anon-volumetric representation thereof. The rendering of the light fieldbased on the 2D rectangular grid of 2D videos may be considered areconstruction of the light field from its non-volumetricrepresentation.

Various other types of volumetric videos exist, as well as conversiontechniques to obtain a non-volumetric representation thereof andreconstruction techniques to reconstruct the volumetric video from itsnon-volumetric representation.

Identifying the Spatial Subregion

The spatial subregion of the non-volumetric video which is occluded bythe volumetric video during display may be detected in various ways. Forexample, if both videos are rendered in a 3D graphics-based environment,occlusion may be detected using known 3D graphics culling techniques.For example, a common method for performing 3D graphics culling uses amixed GPU/CPU approach to implement the Hierarchical Z-Buffer (HZB)occlusion culling algorithm, e.g., as described in the publication“Hierarchical Z-Buffer Visibility” by Ned Greene et al., 1993. Theoutput of the HZB occlusion culling algorithm may be regarded as agrid-based representation of the output buffer (e.g., the screen orwindow) where for each pixel it is indicated whether it is occluded ornot. To determine whether, and if so, which parts of a video in the 3Dgraphics-based environment are occluded, the pixels corresponding to thebounding area (e.g., bounding box or sphere) of the video may beconsidered in the HZB occlusion culling algorithm, while disregardingall pixels outside this bounding area. Next, a polygon may bereconstructed of the occluded area indicated by the HZB cullingalgorithm (e.g. using Chan's algorithm as known from the field ofcomputational geometry). This polygon may be used as a basis foridentifying the spatial subregion in which the data of thenon-volumetric data is to be replaced. In some embodiments, the polygonmay also be included in the signaling data to identify the spatialsubregion.

Another option is that raytracing techniques may be used, in which it isdetected which parts of objects are not hit by viewing rays andtherefore are determined to be occluded. In general, various types ofdata characterizing the relationship between the non-volumetricbackground video and the volumetric foreground video may be used todetermine which part of the non-volumetric background video is occluded.It is noted that such data may be present at a processor systemrepresenting the client device, but in some embodiments also at anotherentity, such as a processor system generating the composite videostream. For example, the latter processor system may be aware of therelation between the non-volumetric video and the volumetric video as itmay, at least in part, determine this relation, for example in aclient-server context in which a server knows the geometry of the scenerendered by a client device. Another example is that the processorsystem generating the composite video stream may obtain this data assignaling data from the client device.

In some embodiments, the display position of the volumetric videorelative to the non-volumetric video may be predetermined. Such apredetermined display position may directly or indirectly indicate thespatial subregion which is occluded by the volumetric video duringdisplay. For example, in multi-user communication, including theaforementioned Social VR use cases, volumetric foreground videos may beinserted at particular positions relative to a non-volumetric backgroundvideo. These positions may also be referred to as ‘placement positions’,and may indicate which spatial subregion of the non-volumetricbackground video is occluded. Such placement position of the volumetricvideo may be defined by metadata, which is also referred to as placementmetadata. The placement metadata may be associated with thenon-volumetric video. For example, the placement metadata may be part ofa same data container as the non-volumetric video, for example a samefile or media stream, but may also be provided as separate metadatawhich can be associated with the non-volumetric video. For example, themetadata may contain an identifier of the data container of thenon-volumetric video, such as an URL, thereby allowing the metadata tobe retrieved and associated with the background video. Yet anotherexample is that the metadata may be included in a manifest file which isassociated with the non-volumetric video, or that it may be included ina service announcement.

It is noted that the spatial subregion which is occluded may bepredicted, in that occlusion may not yet occur but may be predicted tooccur in the (near) future.

Signaling Data

There are various options for generating signaling data which isindicative of the composite video stream containing the non-volumetricrepresentation of the volumetric video. The generated signaling data maythereby directly or indirectly indicate that the composite video streamcontains the non-volumetric representation of the volumetric video. Forexample, Supplemental Enhancement Information (SEI) messages may be usedto, on a tile-by-tile basis, signal which type of data a particular tilecontains, e.g., data of the non-volumetric video or data of thenon-volumetric representation of the volumetric video. Instead offurther referring to tiles, the following examples refer to a ‘spatialregion’. It will be appreciated that such a spatial region may be orcomprise one or more spatial segments, or specifically one or moretiles.

SEI messages may be compatible with HEVC decoders, and may thus becombined with a region-based approach to generate the composite videobitstream. In a specific example, a ‘volumetric region content typedescription’ may be included in a SEI message for specific regions thatcontain volumetric video data. For example, the SEI message may bedefined to contain a content type identifier identifying a region'scontent as either 1) 2D video, 2) point cloud, 3) light field or 4)other. Regions of content types 2-4 may be further described withmetadata, for example containing depth, texture or color or an occupancymap.

An example of a SEI message syntax may be the following:

Descriptor region_content_type(payloadSize ) {   region_type ue(v)  top_left_corner ue(v)   bottom_right_corner ue(v)  if(content_type ==2) {  pcc_content_type u(5)  } }

wherein:

-   -   region_type: describes the region content type (e.g. 2D video,        point cloud, light field, other).    -   top left corner: top left position in luma sample coordinate of        data with respect to the width and the height of the video        sequence    -   bottom_right_corner bottom right position in luma sample        coordinate of data with respect to the width and the height of        the video sequence    -   pcc_content_type: metadata that describes the type of point        cloud data.

It will be appreciated that the descriptor field sizes described in thistable are mentioned as examples and may differ depending on theapplication.

Another example is the use of Network Abstraction Layer (NAL)information, for example using header information as described withtable 11 of WO2018/011042 A1 (herewith incorporated by reference in asfar as pertaining to the syntax of the nal_unit_header) but with the TPSNAL unit with a given nuh_tile_id providing the properties of the giventile content type unit (instead of, as described by WO2018/011042A1,providing properties of a given tile positioning unit).

In case of NAL, the SEI message syntax may be the following:

Descriptor region_content_type(payloadSize ) {  region_type ue(v) tile_id ue(v)  if(content_type == 2) {   pcc_content_type u(5)  } }

wherein:

-   -   Tile_id: value corresponding to the ‘nuh_tile_id’ of the tile        that the SEI message describes (tile as defined in the context        of WO2018/011042 A1)

Yet another option is including the signaling information in a manifestof a tiled composited video stream, such as a MPEG DASH SRD manifest. Asan example, the processor system generating the composite video streammay receive or have access to a manifest of the non-volumetric video anda manifest of the non-volumetric representation of the volumetric video,e.g., as respective manifest files. The latter manifest may have beengenerated by the processor system when generating the non-volumetricrepresentation of the volumetric video, or may have been generated byanother entity and may be accessed by the processor system. Examples ofsuch manifests include, but are not limited to, MPEG DASH SRD manifestfiles. From these manifests, a modified manifest may be created whichmay include references to spatial segments that contain the non-occludedparts of the non-volumetric video and references to spatial segmentsthat contain the non-volumetric representation of the volumetric video.In such a manifest, spatial segments from the non-volumetric video whichare occluded by the volumetric video during rendering may not be listed.

General

In general, for example in multi-user communication, a volumetric videomay be obtained by a 2D camera and a depth camera or by a 3D camera.

In multi-user communication, the functionality of the processor systemgenerating the composite video stream may be implemented by one of theclient devices. Effectively, such a client device may also ‘act’ asserver.

In some embodiments, the non-volumetric video and the non-volumetricrepresentation of the volumetric video may be tiled separately, e.g. inthe uncompressed domain, occlusion detection may be performed, occludedtiles in the non-volumetric video may be detected and replaced by tilesof the non-volumetric representation of the volumetric video, and jointencoding of the resulting tiled video frames may take place. In someembodiments, a spatial dimension of the tiled non-volumetric video maybe extended so as to enable all tiles of the non-volumetricrepresentation of the volumetric video to be included in the resultingvideo stream.

FIG. 6 shows a processor system 600 for generating a composite videostream. For that purpose, the processor system 600 is shown to comprisea processor 630 which may be configured, e.g., by hardware design orsoftware, to perform operations described with reference to FIGS. 1A-5and elsewhere pertaining to the generating of a composite video streamand the generating of the signaling data. For example, the processor 630may be embodied by a single Central Processing Unit (CPU), but also by acombination or system of such CPUs and/or other types of processingunits. The processor system 600 is further shown to comprise a datastorage 625, such as internal memory, a hard disk, a solid-state drive,or an array thereof, which may be used to store or buffer data such asthe volumetric video and the non-volumetric video, and/or to buffer anyreceived video streams representing said videos. The processor system600 is further shown to comprise an input interface 620 to the datastorage 625, which may be any suitable type of interface, e.g., aninternal storage interface such as a memory interface or a solid-statedrive interface. In some embodiments, the data storage 625 may be anexternal data storage 625, and the input interface 620 may be aninterface to the external storage, such as a USB interface or a networkinterface. In some embodiments, the input interface 620 may be the samenetwork interface as the network interface 610 described in thefollowing.

FIG. 6 further shows the processor system 600 to comprise a networkinterface 610, which may be any suitable type of network interface viawhich the composite video stream and the signaling data may betransmitted, both types of data being indicated by reference numeral612. For example, the network interface 610 may be a wireless networkinterface, e.g., based on Wi-Fi, Bluetooth, ZigBee, 4G or 5G mobilecommunication, or a wired network interface, e.g., based on Ethernet oroptical fiber. For example, the network interface 610 may be a localarea network (LAN) network interface or an interface to wide areanetwork (WAN) such as the Internet.

The processor system 600 may be embodied by a (single) device orapparatus. For example, the processor system 600 may be embodied by aserver, workstation, personal computer, etc. In some embodiments, theprocessor system 600 may be an end-user device, for example (integratedinto) a same type of device as described with reference to FIG. 7 whichis configured for rendering the composite video stream. Examples of suchdevices include, but are not limited to a smartphone, personal computer,laptop, tablet device, gaming console, set-top box, television, monitor,projector, smart watch, smart glasses, media player, media recorder,head mounted display device, etc. The processor system 600 may also beembodied by a distributed system of such devices or apparatuses. Anexample of the latter may be the functionality of the processor system600 being at least in part distributed over network elements in anetwork. In another example, the processor system 600 may be embodied byan edge node of a 5G or next-gen telecommunication network, or ingeneral by a network processing node of a telecommunication network.

FIG. 7 shows a processor system 700 configured for rendering thecomposite video stream. The processor system 700 may implement part orall of the ‘decode’, ‘render’ and/or ‘display’ functionality asdescribed with reference to FIGS. 1-5 and elsewhere. The processorsystem 700 is shown to comprise a network interface 710 which may beconfigured to receive a composite video stream and signaling data, bothtypes of data being indicated by reference numeral 712. The networkinterface 710 may be any suitable type of interface for receiving and/ortransmitting said data, including but not limited to a type of networkinterface as described with reference to FIG. 6 . The processor system700 may further comprise a processor 720 which may be configured, e.g.,by hardware design or software, to perform operations described withreference to FIGS. 1A-5 and elsewhere pertaining to the decoding andrendering of the composite video stream. In some embodiments, theprocessor 720 may directly generate and output display data 732representing rendered image data to a display 735 such as an HMD. Inother embodiments, the processor 720 may output rendered image datawhich may be output to the display 735 by an output interface 730.

The processor 720 may be embodied by a single Central Processing Unit(CPU), but also by a combination or system of such CPUs and/or othertypes of processing units, such as Graphics Processing Units (GPUs).Although not shown in FIG. 7 , the processor system 700 may alsocomprise a data storage, such as internal memory, a hard disk, asolid-state drive, or an array thereof, which may be used to bufferdata, e.g., the received composite video stream and/or the signalingdata. The processor system 700 may be embodied by a (single) device orapparatus. For example, the processor system 700 may be embodied assmartphone, personal computer, laptop, tablet device, gaming console,set-top box, television, monitor, projector, smart watch, smart glasses,media player, media recorder, head mounted display device, etc. Theprocessor system 700 may also be embodied by a distributed system ofsuch devices or apparatuses.

In general, the processor system 600 of FIG. 6 and the processor system700 of FIG. 7 may each be embodied as, or in, a device or apparatus. Thedevice or apparatus may comprise one or more (micro)processors whichexecute appropriate software. The processors of either system may beembodied by one or more of these (micro)processors. Softwareimplementing the functionality of either system may have been downloadedand/or stored in a corresponding memory or memories, e.g., in volatilememory such as RAM or in non-volatile memory such as Flash.Alternatively, the processors of either system may be implemented in thedevice or apparatus in the form of programmable logic, e.g., as aField-Programmable Gate Array (FPGA). Any input and/or output interfacesmay be implemented by respective interfaces of the device or apparatus,such as a network interface. In general, each unit of either system maybe implemented in the form of a circuit. It is noted that either systemmay also be implemented in a distributed manner. e.g., involvingdifferent devices.

FIG. 8 shows a computer-implemented method 800 for generating acomposite video stream. The method 800 may comprise, in a step titled“OBTAINING NON-VOLUMETRIC VIDEO AND VOLUMETRIC VIDEO”, obtaining 810 anon-volumetric video, as well as a volumetric video, at least part ofwhich is to be displayed by the client device in front of thenon-volumetric video. The method 800 may further comprise, in a steptitled “DETERMINING SPATIAL SUBREGION IN NON-VOLUMETRIC VIDEO”,determining 820 a spatial subregion of the non-volumetric video which ispartially or entirely occluded when the volumetric video is displayed bythe client device in front of the non-volumetric video. The method 800may further comprise, in a step titled “GENERATING COMPOSITE VIDEOSTREAM”, generating 830 a composite video for the client device by, forrespective input frames of the non-volumetric video and the volumetricvideo and in a step titled “GENERATING NON-VOLUMETRIC REPRESENTATION OFVOLUMETRIC VIDEO”, generating 832 a non-volumetric representation of thevolumetric video using a conversion technique which allows thevolumetric video to be reconstructed from the non-volumetricrepresentation, and in a step titled “REPLACING DATA OF SPATIALSUBREGION OF NON-VOLUMETRIC VIDEO BY DATA OF NON-VOLUMETRICREPRESENTATION OF VOLUMETRIC VIDEO”, replacing 834 data of the spatialsubregion of the non-volumetric video by data of the non-volumetricrepresentation of the volumetric video, thereby obtaining an outputframe of the composite video. The method 800 may further comprise, in astep titled “STREAMING COMPOSITE VIDEO STREAM”, streaming 840 thecomposite video as a composite video stream, or streaming select spatialsegments of the composite video stream, to the client device. The method800 may further comprise, in a step titled “PROVIDING SIGNALING DATA”,providing 845 signaling data to the client device which is indicative ofthe composite video stream containing the non-volumetric representationof the volumetric video.

FIG. 9 shows a computer-implemented method 850 for rendering avolumetric video in front of a non-volumetric video. The method 850 maycomprise, in a step titled “RECEIVING COMPOSITE VIDEO STREAM”, receiving860 a composite video stream containing the non-volumetric video, orselect spatial segments of the composite video stream. The method 850may further comprise, in a step titled “RECEIVING SIGNALING DATA”,receiving 885 signaling data which is indicative of the composite videostream containing a non-volumetric representation of the volumetricvideo in a spatial subregion of the non-volumetric video. The method 850may further comprise, in a step titled “RENDERING COMPOSITE VIDEOSTREAM”, rendering 870 the composite video stream by, for a respectiveinput frame of the composite video stream and in a step titled “DECODINGCOMPOSITE VIDEO STREAM”, decoding 872 the composite video stream, or theselect spatial segments of the composite video stream. The rendering 870may further comprise, in a step titled “IDENTIFYING NON-VOLUMETRICREPRESENTATION OF VOLUMETRIC VIDEO”, on the basis of the signaling data,identifying 874 the non-volumetric representation of the volumetricvideo in the input frame, and in a step titled “RECONSTRUCTINGVOLUMETRIC VIDEO”, reconstructing 876 the volumetric video from thenon-volumetric representation of the volumetric video using areconstruction technique. The rendering 870 may further comprise, in astep titled “RENDERING VOLUMETRIC VIDEO IN FRONT OF NON-VOLUMETRICVIDEO”, rendering 878 the volumetric video in front of thenon-volumetric video so that the spatial subregion of the non-volumetricvideo is partially or entirely occluded by the volumetric video.

It will be appreciated that, in general, the operations of method 800 ofFIG. 8 and/or method 850 of FIG. 9 may be performed in any suitableorder, e.g., consecutively, simultaneously, or a combination thereof,subject to, where applicable, a particular order being necessitated,e.g., by input/output relations.

It is noted that any of the methods described in this specification, forexample in any of the claims, may be implemented on a computer as acomputer implemented method, as dedicated hardware, or as a combinationof both. Instructions for the computer, e.g., executable code, may bestored on a computer readable medium 900 as for example shown in FIG. 10, e.g., in the form of a series 910 of machine-readable physical marksand/or as a series of elements having different electrical, e.g.,magnetic, or optical properties or values. The executable code may bestored in a transitory or non-transitory manner. Examples of computerreadable mediums include memory devices, optical storage devices,integrated circuits, servers, online software, etc. FIG. 10 shows by wayof example an optical storage device 900.

In an alternative embodiment of the computer readable medium 900 of FIG.10 , the computer readable medium 900 may comprise transitory ornon-transitory data 910 represent the signaling data or placementmetadata described in this specification.

FIG. 11 is a block diagram illustrating an exemplary data processingsystem 1000 that may be used in the embodiments described in thisspecification. Such data processing systems include data processingentities described in this specification, including but not limited tothe processor systems, servers and client devices as described withreference to FIGS. 1-7 and elsewhere, and others.

The data processing system 1000 may include at least one processor 1002coupled to memory elements 1004 through a system bus 1006. As such, thedata processing system may store program code within memory elements1004. Furthermore, processor 1002 may execute the program code accessedfrom memory elements 1004 via system bus 1006. In one aspect, dataprocessing system may be implemented as a computer that is suitable forstoring and/or executing program code. It should be appreciated,however, that data processing system 1000 may be implemented in the formof any system including a processor and memory that is capable ofperforming the functions described within this specification.

The memory elements 1004 may include one or more physical memory devicessuch as, for example, local memory 1008 and one or more bulk storagedevices 1010. Local memory may refer to random access memory or othernon-persistent memory device(s) generally used during actual executionof the program code. A bulk storage device may be implemented as a harddrive, solid state disk or other persistent data storage device. Thedata processing system 1000 may also include one or more cache memories(not shown) that provide temporary storage of at least some program codein order to reduce the number of times program code is otherwiseretrieved from bulk storage device 1010 during execution.

Input/output (I/O) devices depicted as input device 1012 and outputdevice 1014 optionally can be coupled to the data processing system.Examples of input devices may include, but are not limited to, forexample, a microphone, a keyboard, a pointing device such as a mouse, agame controller, a Bluetooth controller, a VR controller, and agesture-based input device, or the like. Examples of output devices mayinclude, but are not limited to, for example, a monitor or display,speakers, or the like. Input device and/or output device may be coupledto data processing system either directly or through intervening 1/Ocontrollers. A network adapter 1016 may also be coupled to dataprocessing system to enable it to become coupled to other systems,computer systems, remote network devices, and/or remote storage devicesthrough intervening private or public networks. The network adapter maycomprise a data receiver for receiving data that is transmitted by saidsystems, devices and/or networks to said data and a data transmitter fortransmitting data to said systems, devices and/or networks. Modems,cable modems, and Ethernet cards are examples of different types ofnetwork adapter that may be used with data processing system 1000.

As shown in FIG. 11 , memory elements 1004 may store an application1018. It should be appreciated that data processing system 1000 mayfurther execute an operating system (not shown) that can facilitateexecution of the application. The application, being implemented in theform of executable program code, can be executed by data processingsystem 1000, e.g., by processor 1002. Responsive to executing theapplication, the data processing system may be configured to perform oneor more operations to be described herein in further detail.

For example, data processing system 1000 may represent a processorsystem or entity configured for generating the composite video stream,e.g., as described with reference to FIGS. 1-6 . In that case,application 1018 may represent an application that, when executed,configures data processing system 1000 to perform the functionsdescribed with reference to said processor system or entity.

In another example, data processing system 1000 may represent aprocessor system or entity configured for rendering the composite videostream, e.g., as described with reference to FIGS. 1-7 . In that case,application 1018 may represent an application that, when executed,configures data processing system 1000 to perform the functionsdescribed with reference to said processor system and/or entity.

In the claims, any reference signs placed between parentheses shall notbe construed as limiting the claim. Use of the verb “comprise” and itsconjugations does not exclude the presence of elements or steps otherthan those stated in a claim. Expressions such as “at least one of” whenpreceding a list or group of elements represent a selection of all or ofany subset of elements from the list or group. For example, theexpression, “at least one of A, B, and C” should be understood asincluding only A, only B, only C, both A and B, both A and C, both B andC, or all of A, B, and C. The article “a” or “an” preceding an elementdoes not exclude the presence of a plurality of such elements. Theinvention may be implemented by means of hardware comprising severaldistinct elements, and by means of a suitably programmed computer. Inthe device claim enumerating several means, several of these means maybe embodied by one and the same item of hardware. The mere fact thatcertain measures are recited in mutually different dependent claims doesnot indicate that a combination of these measures cannot be used toadvantage.

The invention claimed is:
 1. A processor system configured forgenerating a composite video stream for transmission to a client device,the processor system comprising: a network interface to a network; aninput interface for obtaining: a non-volumetric video; and a volumetricvideo, at least part of which is to be displayed by the client device infront of the non-volumetric video; a processor configured to: determinea spatial subregion of the non-volumetric video which is partially orentirely occluded when the volumetric video is displayed by the clientdevice in front of the non-volumetric video; and generate a compositevideo for the client device by, for respective input frames of thenon-volumetric video and the volumetric video: obtaining anon-volumetric representation of the volumetric video which is generatedusing a conversion technique for enabling the volumetric video to bereconstructed from the non-volumetric representation; replacing data ofthe spatial subregion of the non-volumetric video by data of thenon-volumetric representation of the volumetric video, thereby obtainingan output frame of the composite video; and via the network interface:stream the composite video as a composite video stream, or stream selectspatial segments of the composite video stream, to the client device;and provide signaling data to the client device that is indicative ofthe composite video stream containing the non-volumetric representationof the volumetric video.
 2. The processor system according to claim 1,wherein the non-volumetric video is obtained as, or converted into, aspatially segmented encoding comprising independently decodablesegments, and wherein the processor is further configured to: determinethe spatial subregion as a set of segments of the non-volumetric videowhich are occluded when the volumetric video is displayed by the clientdevice in front of the non-volumetric video; encode the non-volumetricrepresentation of the volumetric video as one or more independentlydecodable segments; and remove the set of segments from the spatiallysegmented encoding of the non-volumetric video, and add the one or moresegments to the spatially segmented encoding of the non-volumetricvideo, to obtain a spatially segmented encoding of the output frame ofthe composite video stream.
 3. The processor system according to claim2, wherein the processor is further configured to generate the signalingdata to identify the set of segments as containing the non-volumetricrepresentation of the volumetric video.
 4. The processor systemaccording to claim 3, wherein the processor is further configured toinclude the signaling data in the composite video stream, for example asa Supplemental Enhancement Information (SEI) message.
 5. The processorsystem according to claim 1, wherein the client device is configured torender the composite video stream in a Virtual Reality (VR) or AugmentedReality (AR) environment and to render the VR/AR environment from aviewing position of a user, wherein the processor is further configuredto: determine the viewing position of the user by receiving dataindicative of the viewing position from the client device; and determinethe spatial subregion by determining which spatial subregion of thevolumetric video is partially or entirely occluded by the volumetricvideo when the volumetric video is displayed in front of thenon-volumetric video in the VR/AR environment and rendered by the clientdevice from the viewing position.
 6. The processor system according toclaim 1, wherein the volumetric video is a 3D point cloud, and whereinthe conversion technique by which the non-volumetric representation ofthe volumetric video is generated is a point cloud compressiontechnique.
 7. The processor system according to claim 1, wherein thenon-volumetric video is a panoramic or omnidirectional video.
 8. Theprocessor system of claim 1, wherein the signaling data indicative ofthe composite video stream containing a non-volumetric representation ofa volumetric video is stored in a non-transitory computer-readablemedium.
 9. The processor system of claim 8, wherein the composite streamis a spatially segmented encoding which comprises independentlydecodable segments, and wherein the signaling data identifies a set ofsegments of the spatially segmented encoding.
 10. A processor systemrepresenting a client device configured to render a volumetric video infront of a non-volumetric video, the processor system comprising: anetwork interface to a network; a processor configured to, via thenetwork interface: receive a composite video stream containing thenon-volumetric video, or select spatial segments of the composite videostream; receive signaling data that is indicative of the composite videostream containing a non-volumetric representation of the volumetricvideo in a spatial subregion of the non-volumetric video; the processorbeing further configured to render the composite video stream by, for arespective input frame of the composite video stream: decoding thecomposite video stream, or the select spatial segments of the compositevideo stream; on the basis of the signaling data, identifying thenon-volumetric representation of the volumetric video in the inputframe; reconstructing the volumetric video from the non-volumetricrepresentation of the volumetric video using a reconstruction technique;and rendering the volumetric video in front of the non-volumetric videoso that the spatial subregion of the non-volumetric video is partiallyor entirely occluded by the volumetric video.
 11. The processor systemaccording to claim 10, wherein: the composite video stream is receivedas a spatially segmented encoding which comprises independentlydecodable segments; and the processor is further configured to identifythe non-volumetric representation of the volumetric video in thecomposite video stream based on the signaling data identifying a set ofsegments of the spatially segmented encoding.
 12. The processor systemaccording to claim 10, wherein the processor is further configured to:render the composite video stream in a Virtual Reality (VR) or AugmentedReality (AR) environment; and render the VR/AR environment from aviewing position of a user.
 13. The processor system according to anyone of claim 10, wherein the volumetric video is a 3D point cloud, andwherein the conversion technique by which the non-volumetricrepresentation of the volumetric video is generated is a point cloudcompression technique.
 14. The processor system according to any one ofclaim 10, wherein the non-volumetric video is a panoramic oromnidirectional video.
 15. A computer-implemented method for generatinga composite video stream for transmission to a client device, the methodcomprising: obtaining: a non-volumetric video; and a volumetric video,at least part of which is to be displayed by the client device in frontof the non-volumetric video; determining a spatial subregion of thenon-volumetric video which is partially or entirely occluded when thevolumetric video is displayed by the client device in front of thenon-volumetric video; generating a composite video for the client deviceby, for respective input frames of the non-volumetric video and thevolumetric video: obtaining a non-volumetric representation of thevolumetric video which is generated using a conversion technique whichallows the volumetric video to be reconstructed from the non-volumetricrepresentation; replacing data of the spatial subregion of thenon-volumetric video by data of the non-volumetric representation of thevolumetric video, thereby obtaining an output frame of the compositevideo; streaming the composite video as a composite video stream, orstreaming select spatial segments of the composite video stream, to theclient device; and providing signaling data to the client device that isindicative of the composite video stream containing the non-volumetricrepresentation of the volumetric video.
 16. A computer-implementedmethod for rendering a volumetric video in front of a non-volumetricvideo, the method comprising: receiving a composite video streamcontaining the non-volumetric video, or select spatial segments of thecomposite video stream; receiving signaling data that is indicative ofthe composite video stream containing a non-volumetric representation ofthe volumetric video in a spatial subregion of the non-volumetric video;rendering the composite video stream by, for a respective input frame ofthe composite video stream: decoding the composite video stream, or theselect spatial segments of the composite video stream; on the basis ofthe signaling data, identifying the non-volumetric representation of thevolumetric video in the input frame; reconstructing the volumetric videofrom the non-volumetric representation of the volumetric video using areconstruction technique; and rendering the volumetric video in front ofthe non-volumetric video so that the spatial subregion of thenon-volumetric video is partially or entirely occluded by the volumetricvideo.
 17. A non-transitory computer-readable medium having instructionsstored thereon for generating a composite video stream for transmissionto a client device, wherein the instructions, when executed by aprocessor of a processor system, cause the processor system to carry outoperations including: obtaining: a non-volumetric video; and avolumetric video, at least part of which is to be displayed by theclient device in front of the non-volumetric video; determining aspatial subregion of the non-volumetric video which is partially orentirely occluded when the volumetric video is displayed by the clientdevice in front of the non-volumetric video; generating a compositevideo for the client device by, for respective input frames of thenon-volumetric video and the volumetric video: obtaining anon-volumetric representation of the volumetric video which is generatedusing a conversion technique which allows the volumetric video to bereconstructed from the non-volumetric representation; replacing data ofthe spatial subregion of the non-volumetric video by data of thenon-volumetric representation of the volumetric video, thereby obtainingan output frame of the composite video; streaming the composite video asa composite video stream, or streaming select spatial segments of thecomposite video stream, to the client device; and providing signalingdata to the client device that is indicative of the composite videostream containing the non-volumetric representation of the volumetricvideo.
 18. A non-transitory computer-readable medium having instructionsstored thereon for rendering a volumetric video in front of anon-volumetric video, wherein the instructions, when executed by aprocessor of a client device, cause the client device to carry outoperations including: receiving a composite video stream containing thenon-volumetric video, or select spatial segments of the composite videostream; receiving signaling data that is indicative of the compositevideo stream containing a non-volumetric representation of thevolumetric video in a spatial subregion of the non-volumetric video;rendering the composite video stream by, for a respective input frame ofthe composite video stream: decoding the composite video stream, or theselect spatial segments of the composite video stream; on the basis ofthe signaling data, identifying the non-volumetric representation of thevolumetric video in the input frame; reconstructing the volumetric videofrom the non-volumetric representation of the volumetric video using areconstruction technique; and rendering the volumetric video in front ofthe non-volumetric video so that the spatial subregion of thenon-volumetric video is partially or entirely occluded by the volumetricvideo.